The slew rate of a signal path of an electronic circuit determines how fast a signal carried by the signal path can transition from a first state to a second state. These two states are often referred to as ‘OFF’ and ‘ON’, or ‘0’ and ‘1’. The value of the slew rate must be carefully chosen, because a slew rate that is too fast may cause unacceptable circuit ringing and degrade the signal quality. Conversely, a slew rate that is too slow may not meet the design specifications of the electronic circuit. A determination of how fast is too fast or how slow is too slow depends upon the characteristics of the circuit and its intended application.
Control of the slew rate may be achieved by controlling the impedance of a subcircuit containing the signal path. This subcircuit may be the driver circuit of an integrated circuit, for example. Referring now to FIG. 1, a driver slew rate control circuit that controls the slew rate by changing an impedance of the control circuit is shown, according to the prior art. Impedance control circuitry 115 uses off-chip Process-Voltage-Temperature (PVT) information 135 and on-chip PVT information to determine an amount of impedance to apply to output data 105. Pre-driver circuitry 120 uses an output enable signal 110 to determine when output data 105 requires slew rate control. Pre-driver circuitry 120 is coupled to transistors (represented as transistors 145, 150, 155) and pull-down transistors (represented as transistors 160, 165, 170). The pull-up transistors (145, 150, 155) and the pull-down transistors (160, 165, 170) operate on output data 105 to produce output signal 175 with a specified slew rate performance. The slew rate performance is determined by one or more circuit specifications and the off-chip PVT information 135 and on-chip PVT information 140. Note that the complexity of the slew rate control is compounded by the use of impedance control using the pull-up transistors (145, 150, 155) and the pull-down transistors (160, 165, 170). Impedance control requires the off-chip PVT information 135 and on-chip PVT information 140 since the pull-up transistors (145, 150, 155) and the pull-down transistors (160, 165, 170) are affected by variations in process of manufacture, temperature and voltage.